1. Field of the Invention
The present invention relates to a piezoelectric thin film comprising a piezoelectric layer. The present invention further relates to an ink jet head comprising the piezoelectric thin film, a method for forming an image with the head, an angular velocity sensor comprising the piezoelectric thin film, a method for measuring an angular velocity with the sensor, a piezoelectric generating element comprising the piezoelectric thin film and a method for generating electric power with the element.
2. Description of Related Art
Lead zirconate titanate (PZT: Pb(ZrxTi1-x)O3, 0<x<1) is a typical ferroelectric material capable of storing a large amount of electric charge. PZT is used in capacitors and thin film memories. PZT has pyroelectricity and piezoelectricity based on the ferroelectricity thereof. PZT has high piezoelectric performance. The mechanical quality factor Qm of PZT can be controlled easily by adjusting the composition or adding an element thereto. These facts allow PZT to be applied to sensors, actuators, ultrasonic motors, filter circuits and oscillators.
PZT, however, contains a large amount of lead. In recent years, there has been a growing concern that lead leaching from waste may cause serious damage to the ecosystem and the environment. Accordingly, there has been an international movement to restrict the use of lead. For this reason, ferroelectric materials containing no lead (lead-free ferroelectric materials) unlike PZT have been in demand.
One example of the lead-free ferroelectric materials that are currently under development is a ceramic of a perovskite composite oxide [(Bi0.5Na0.5)1-yBay]TiO3 composed of bismuth (Bi), sodium (Na), barium (Ba) and titanium (Ti). JP 4(1992)-60073 B and T. Takenaka et al., Japanese Journal of Applied Physics, Vol. 30, No. 9B, (1991), pp. 2236-2239, disclose that this ferroelectric material has a piezoelectric constant d33 of about 125 pC/N and thus exhibits high piezoelectric performance, when the content of barium y (=[Ba/(Bi+Na+Ba)] is 5% to 10%.
In bulk ferroelectrics such as a ceramic, attempts have been made to improve the piezoelectric performance by adding a slight amount of dopant. D. Q. Xiao et al., Japanese Journal of Applied Physics, Vol. 44, No. 12, (2005), pp. 8515-8518, discloses a (Bi, Na, Ba)TiO3 ceramic to which silver (Ag) is added ((Bi, Na, Ba, Ag)TiO3 ceramic). The piezoelectric constant of this ceramic increases more, as the additional amount of Ag increases. Particularly, the (Bi, Na, Ba, Ag)TiO3 ceramic having a composition with a mole ratio of Ag to Ti of 0.03 has a piezoelectric constant d33 that is about 9% higher than the piezoelectric constant d33 of the (Bi, Na, Ba, Ag)TiO3 ceramic free from Ag. However, with the addition of Ag, the dielectric loss tan δ of the ceramic increases and the electromechanical coupling coefficient decreases. The electromechanical coupling coefficient represents the conversion efficiency of the electrical energy and mechanical energy.
Other attempts have been made to improve the piezoelectric performance in a piezoelectric thin film by aligning the orientation axis of the crystal in the thin film. One of the methods is to arrange an interface layer (buffer layer) between a substrate and a piezoelectric thin film. JP 10 (1998)-182291 A and JP 2007-266346 A disclose the interface layer containing all or some of the constituent elements of the piezoelectric thin film. Independently of this, H. W. Cheng et al., Applied Physics Letters, Vol. 85, No. 12, (2004), pp. 2319-2321, and Z. H. Zhou et al., Applied Physics Letters, Vol. 85, No. 5, (2004), pp. 804-806, disclose the formation of a piezoelectric thin film oriented in a specific direction by pulsed laser deposition (PLD) or RF magnetron sputtering.
A piezoelectric thin film in which the crystal orientation is not controlled exhibits very low ferroelectric performance and piezoelectric performance. The ferroelectric performance, for example, is the remanent polarization. (Bi, Na, Ba)TiO3 in the form of a ceramic shows a dielectric loss of about 1% (see T. Takenaka et al., Japanese Journal of Applied Physics, Vol. 30, No. 9B, (1991), pp. 2236-2239). In contrast, particularly in a low frequency region of 1 kHz or less, the dielectric loss of the (Bi, Na, Ba)TiO3 thin film suddenly increases and reaches several tens % (see Z. H. Zhou et al., Applied Physics Letters, Vol. 85, No. 5, (2004), pp. 804-806). This is because a leak current is likely to occur in the (Bi, Na, Ba)TiO3 thin film. When the dielectric loss is high, the ferroelectric performance and piezoelectric performance of the thin film is considerably low.
JP 2008-169113 A discloses a piezoelectric magnetic composition that contains Ag as an additional element and is expressed by a specific chemical formula. This composition is a bulk. This composition contains no lead and is excellent in at least one property selected from piezoelectric constant d31, relative dielectric constant, dielectric loss and Curie temperature Tc.